1. Field of the Invention
The present invention relates to a dynamic microphone unit and a dynamic microphone that can achieve reduction of resonance so as to improve directivity.
2. Description of the Related Art
An example of a conventional unidirectional dynamic microphone unit shown in FIG. 6 will first be described. In FIG. 6, the dynamic microphone unit includes a unit case 9 as a base, and is configured by installing the following members in the unit case 9.
The unit case 9 has a cylindrical shape and includes a flange 91 formed on an upper end portion thereof and protruding inward, and an outer periphery surface of a ring yoke 1 having a cylindrical shape is fitted and fixed to an inner periphery surface of the flange 91. The ring yoke 1 is made of a magnetic substance. A disk-shaped yoke 2 is fixed to the ring yoke 1 by being pushed against a step portion that is formed over the entire circumference of an inner periphery side of the ring yoke 1. The yoke 2 and the ring yoke 1 are magnetically coupled. The yoke 2 includes a plurality of holes 22 penetrating through the yoke 2 in a thickness direction, which are formed in a circumferential direction at certain intervals.
To an upper surface of the yoke 2, a disk-shaped magnet 3 is fixed in such a manner that a central axis thereof is identical to a central axis of the yoke 2. An outer diameter of the magnet 3 is defined such that each of holes 22 of the yoke 2 is only partially covered by the magnet 3. To an upper surface of the magnet 3, a disk-shaped pole piece 4 is fixed in such a manner that a central axis thereof is identical to the central axis of the magnet 3.
In an upper end of the ring yoke 1, a flange 11 is formed protruding inward, and an inner periphery surface of the flange 11 faces an outer periphery surface of the pole piece 4 across a relatively narrow gap. The gap is a cylindrical-shaped magnetic gap. The magnet 3, the yoke 2, the ring yoke 1, the magnetic gap, and the pole piece 4 form a magnetic circuit. Magnetic flux emanating from the magnet 3 passes through the magnetic circuit and returns to the magnet 3. In the magnetic gap, a magnetic field is uniformly formed in a circumferential direction.
The outer diameter of the magnet 3 is smaller than a diameter of the inner periphery surface of the ring yoke 1, and an air chamber 10 is produced between an outer periphery surface of the magnet 3 and the inner periphery surface of the ring yoke 1. The air chamber 10 communicates with a rear air chamber 15 to form part of the rear air chamber. The air chamber 10 is called a second air chamber after the rear air chamber 15.
An outer periphery edge of an upper end of the unit case 9 is formed into a dike 94, and an outer periphery edge portion of a diaphragm 5 is fixed to an inner periphery side of the dike 94. The diaphragm 5 is made of a synthetic resin or a metal film, and includes a center dome 51 and a sub-dome 52 that surrounds the center dome 51. The center dome 51 has a shape obtained by cutting out part of a spherical surface. The sub-dome has a partial-arc-shaped cross section, and is successively formed from an outer periphery edge of the center dome 51. An outer periphery edge portion of the sub-dome 52 is fixed to the unit case 9. Upon receiving acoustic waves, the diaphragm 5 can vibrate in a longitudinal direction (vertical direction in FIG. 6) by sound pressure thereof, using the outer periphery edge portion of the sub-dome 52 as a fulcrum.
To the diaphragm 5, a voice coil 6 is fixed along a circular boundary line between the center dome 51 and the sub-dome 52. The voice coil 6 is formed into a cylindrical shape by winding a thin conductive wire, and one end of the cylindrical shape is fixed to the diaphragm 5. The voice coil 6 is positioned in the magnetic gap, and separated from both of the ring yoke 1 and the pole piece 4.
On a front side of the diaphragm 5, an equalizer 8 which also serves as a protecting member for the diaphragm 5 is disposed. An outer periphery edge portion of the equalizer 8 is fixed to the dike 94 of the unit case 9. A ceiling surface in a central portion of the equalizer 8 is formed into a dome shape, and a gap is maintained with respect to the center dome 51 of the diaphragm 5. The equalizer 8 has a plurality of holes for conducting the acoustic waves to the diaphragm 5.
To an outer periphery of a lower end portion of the ring yoke 1, an open end portion of a bottomed cylinder 14 is fitted, and the lower end of the ring yoke 1 is closed by the bottomed cylinder 14. The rear air chamber 15, which is relatively large, is formed inside the ring yoke 1 and the bottomed cylinder 14. In the rear air chamber 15, an acoustic resistor 7 is disposed being in intimate contact with a lower surface of the yoke 2. An upper end surface of the acoustic resistor 7 is pressed against the lower surface of the yoke 2. The acoustic resistor 7 is disposed on a back side of the diaphragm 5. A back side space of the diaphragm 5 communicates with the acoustic resistor 7 through the magnetic gap, the second air chamber 10, and the holes 22 of the yoke 2, and further communicates with the air chamber 15.
In the flange 91 of the unit case 9, an appropriate number of holes 93 penetrating through the flange 91 in a thickness direction are formed. The holes 93 connect a back side space of the sub-dome 52 of the diaphragm 5 with an external space. With such a configuration, directivity of the microphone unit is made unidirectional. On an outer periphery surface side of the unit case 9, a microphone case coupling part 92 is formed. A microphone case is fitted into the microphone case coupling part 92 to form a dynamic microphone.
Upon receiving the acoustic waves, the diaphragm 5 vibrates in the longitudinal direction in accordance with changes in the sound pressure thereof, and the voice coil 6 vibrates in the longitudinal direction together with the diaphragm 5. When the voice coil 6 vibrates, the voice coil 6 traverses the magnetic flux passing through the magnetic gap, and the voice coil 6 generates voice signals in accordance with the changes in the sound pressure. Electroacoustic conversion is performed in such a manner, and the voice signals are output to the outside through leads from both ends of the voice coil 6.
Narrowing the magnetic gap is effective to enhance sensitivity of the dynamic microphone unit, and the magnetic gap is narrowed as possible within such a range as the voice coil 6 is not contact with the pole piece 4 and the ring yoke 1. Thus, the voice coil 6 substantially divides the back side space of the diaphragm 5 into the back side space of the center dome 51 and the back side space of the sub-dome 52. Both of the back side spaces communicate with each other through the magnetic gap.
Here, an acoustic capacitance of the back side space of the center dome 51 is denoted by sc, and an acoustic capacitance of the back side space of the sub-dome 52 is denoted by ss. An acoustic mass of the gap produced between an inner periphery surface of the voice coil 6 and an outer periphery surface of the pole piece 4 is denoted by mgi, and an acoustic resistance thereof is denoted by rgi. An acoustic mass of the gap produced between an outer periphery surface of the voice coil 6 and the inner periphery surface of the flange portion 11 of the ring yoke 1 is denoted by mgo, and an acoustic resistance thereof is denoted by rgo. In addition, a sound pressure applied to the diaphragm 5 from the front side thereof is denoted by P1, an acoustic resistance of the acoustic resistor 7 disposed in the air chamber 15 of the unit case 9 is denoted by r1, an acoustic mass of a back side air chamber of the diaphragm 5 is denoted by m0, and an acoustic capacitance thereof is denoted by s0. Furthermore, an acoustic capacitance of the air chamber 10 produced between an inner periphery wall surface of the yoke 2 and an outer periphery surface of the magnet 3 is denoted by sg, the sound pressure applied to the back side of the diaphragm 5 through the holes 93 is denoted by P2, and an acoustic resistance of the holes 93 is denoted by r2. FIG. 7 shows an equivalent circuit of the conventional dynamic microphone unit including these acoustic capacitances, acoustic masses, and acoustic resistances, as elements thereof.
As shown in FIG. 7, the acoustic capacitances sc and ss of the two back side spaces of the diaphragm 5 are connected via the acoustic mass mgi, the acoustic resistance rgi, the acoustic resistance rgo, and the acoustic mass mgo. The acoustic masses mgi and mgo of the inner periphery side and the outer periphery side of the magnetic gap divided by the voice coil 6, and the acoustic capacitances sc and ss of the back side space of the center dome 51 and the back side space of the sub-dome 52 configure a resonance circuit. Resonance of the resonance circuit makes a frequency response characteristic curve uneven.
In addition, the magnetic gap communicates with the second air chamber 10 that is relatively small as compared with the rear air chamber 15. If a capacity of the second air chamber 10 is small, and therefore the acoustic capacitance sg thereof is small, the resonance due to the acoustic capacitance sg hardly occurs. However, the center dome 51 preferably has a diameter as large as possible because an effective area of the diaphragm 5 is an area of the center dome 51. As the diameter of the center dome 51 is made larger, a diameter of the voice coil 6 is made larger. As the diameter of the voice coil 6 is made larger, diameters of the magnetic gap and the second air chamber 10 are made larger, and the capacity of the second air chamber 10 is made larger, which makes the acoustic capacitance sq of the second air chamber 10 large. As the acoustic capacitance sg is made larger, the resonance due to the acoustic capacitance sg is more likely to occur.
If the resonance is likely to occur in such a manner, peaks develop in the audio frequency band, making frequency response characteristics deteriorate. It is preferable for a unidirectional microphone to have a uniform directivity from low frequency bandwidths to high frequency bandwidths. However, the development of the peaks in the frequency response characteristics makes the directivity non-uniform, causing a problem in which a timbre of acoustic waves in a specific direction changes.
The invention described in Japanese Unexamined Patent Application Publication No. 2013-55466 is disclosed as an art related to the conventional dynamic microphone unit described above. The invention described in Japanese Patent Laid-Open No. 2013-55466 allows for making the acoustic capacitance sg small by making the capacity of the second air chamber 10 small in the dynamic microphone unit including the rear air chamber 15 and the second air chamber 10, in the conventional example shown in FIG. 6. In addition, the dynamic microphone unit has a configuration in which a laminar acoustic resistor is disposed to the second air chamber under a tension, and the voice coil is brought into contact with the acoustic resistor within the maximum displacement thereof. This is for reducing impact noise generated when the diaphragm is widely displaced.